Recovery of data read from memory with unknown polarity

ABSTRACT

A memory controller includes an interface and circuitry. The interface is configured to communicate with a memory device, which includes multiple memory cells, and which applies refreshing to the memory cells by repeatedly inverting data stored in the memory cells. The circuitry is configured to store input data in a group of the memory cells, to read the stored input data from the group of the memory cells to produce read data, the read data has an actual polarity that is either un-inverted or inverted due to the refreshing of the memory cells in the group, to analyze the read data for identifying the actual polarity of the read data, and to recover the input data from the read data based on the identified actual polarity.

TECHNICAL FIELD

Embodiments described herein relate generally to data storage, andparticularly to methods and systems for recovering data read from memorywith unknown polarity.

BACKGROUND

Some types of memory devices refresh the stored data periodically forpreserving the information and improving storage reliability. Memorydevices of certain technologies, such as Ferroelectric Random AccessMemory (FeRAM or FRAM), may suffer from reliability degradation due toan imprint effect, in which the memory cells tend to prefer polarizationstates corresponding to data that has been stored in these cells for along period of time.

Methods for mitigating imprint effects are known in the art. Forexample, U.S. Pat. No. 5,745,403, whose disclosure is incorporatedherein by reference, describes a system and a method for mitigatingundesired imprint effects in a ferroelectric memory array through theaddition of a complementary data path which allows user data to bewritten to the array in an inverted state and then subsequently read outfrom the array in a reinverted state in response to the state of atleast one indicator bit corresponding to each row of the array.

As another example, U.S. Pat. No. 8,495,438, whose disclosure isincorporated herein by reference, describes a method of reducing imprintof a memory cell. The method comprises adding an inversion condition bitoperably associated with one or more memory cells storing a memory word.The inversion condition bit indicates whether the memory word representsan actual payload or an inversion of the actual payload. The inversioncondition bit and memory word are selectively toggled by a controlcircuitry. Inversion is performed by reading the inversion condition bitand memory word and rewriting the memory word back to the one or morememory cells in an inverted or non-inverted state, depending on aninversion condition bit. The inversion condition bit is then written tothe inversion status bit value.

SUMMARY

An embodiment that is described herein provides a memory controller thatincludes an interface and circuitry. The interface is configured tocommunicate with a memory device, which includes multiple memory cells,and which applies refreshing to the memory cells by repeatedly invertingdata stored in the memory cells. The circuitry is configured to storeinput data in a group of the memory cells, to read the stored input datafrom the group of the memory cells to produce read data, the read datahas an actual polarity that is either un-inverted or inverted due to therefreshing of the memory cells in the group, to analyze the read datafor identifying the actual polarity of the read data, and to recover theinput data from the read data based on the identified actual polarity.

In some embodiments, the circuitry is configured to identify the actualpolarity without receiving an explicit indication of the actual polarityfrom the memory device. In other embodiments, the circuitry isconfigured to store the input data after encoding the input data usingan Error Correction Code (ECC) that generates first parity bits andsecond parity bits, such that encoding the input data and an invertedversion of input data results in same first parity bits, but results inmutually-inverted second parity bits. In yet other embodiments, thecircuitry is further configured to read the stored input data includingthe first and second parity bits, to perform at least one of a first ECCdecoding operation that decodes the read data directly, and a second ECCdecoding operation that decodes the read data after inverting the firstparity bits, and to identify the actual polarity of the read data basedon pass/fail outcomes of the at least one of the first and second ECCdecoding operations.

In an embodiment, the circuitry is configured to decide that the actualpolarity is un-inverted when the first ECC decoding operation succeedsand the second ECC decoding operation fails, and to recover the inputdata by outputting the data decoded by the first ECC decoding operation.In another embodiment, the circuitry is configured to decide that theactual polarity is inverted when the second ECC decoding operationsucceeds and the first ECC decoding operation fails, and to recover theinput data by inverting the data decoded by the second ECC decodingoperation. In yet another embodiment, the circuitry is configured toreport a readout failure when both the first ECC decoding operation andthe second ECC decoding operation have failed, or when both the firstECC decoding operation and the second ECC decoding operation havesucceeded. In yet further another embodiment, the circuitry isconfigured to perform the second ECC decoding operation only in responseto a failure of the first ECC decoding operation.

In some embodiments, the memory cells in the group store multiple datapages having respective bit significance values, and the circuitry isconfigured to read the stored input data belonging to one of the datapages stored in the group, and to identify the actual polarity andrecover the stored input data of the one of the data pages. In otherembodiments, the memory cells of the memory device include ferroelectricmemory cells. In yet other embodiments, the circuitry is configured toset a given bit of the input data to a predefined value prior toencoding, to store the input data after encoding the input data using anError Correction Code (ECC) that generates parity bits, such thatencoding the input data and an inverted version of input data results insame values of the parity bits, and to identify the actual polarity byapplying a single decoding operation to the read data to produce decodeddata, and checking the given bit in the decoded data.

There is additionally provided, in accordance with an embodiment that isdescribed herein, a method that includes storing input data in a groupof memory cells of a memory device, which applies refreshing to thememory cells by repeatedly inverting the data stored in the memorycells. The stored input data is read from the group of the memory cellsto produce read data, the read data has an actual polarity that iseither un-inverted or inverted due to the refreshing of the memory cellsin the group. The read data is analyzed to identify the actual polarityof the read data. The input data is recovered from the read data basedon the identified actual polarity.

There is additionally provided, in accordance with an embodiment that isdescribed herein, a storage system that includes a memory and acontroller. The memory includes multiple memory cells and appliesrefreshing to the memory cells by repeatedly inverting data stored inthe memory cells. The controller, is configured to store input data in agroup of the memory cells, to read the stored input data from the groupof the memory cells to produce read data, the read data has an actualpolarity that is either un-inverted or inverted due to the refreshing ofthe memory cells in the group, to analyze the read data to identify theactual polarity of the read data, and to recover the input data from theread data based on the identified actual polarity.

These and other embodiments will be more fully understood from thefollowing detailed description of the embodiments thereof, takentogether with the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram that schematically illustrates a memorysystem, in accordance with an embodiment that is described herein;

FIG. 2 is a block diagram that schematically illustrates an ErrorCorrection Code (ECC) module that recovers data read from a memorydevice with unknown polarity, in accordance with an embodiment that isdescribed herein; and

FIG. 3 is a flow chart that schematically illustrates a method forrecovering data read from a memory device with unknown polarity, inaccordance with an embodiment that is described herein.

DETAILED DESCRIPTION OF EMBODIMENTS Overview

Memories may suffer impairments of various types, depending (among otherfactors) on the underlying technology of the memory cells. For example,data retention in some types of memory cells refers to the ability ofthe memory cells to retain reliable reading of the stored data for longperiods of time. Ferroelectric memory cells may be vulnerable to imprinteffects, in which the memory cells tend to prefer polarization states towhich the memory cell have been programmed for a long period of time.Other impairments comprise, for example, read disturb, program disturb,and/or cosmic radiation that induces bit flipping in the stored data.

One approach to mitigate such impairments is to refresh the stored datarepeatedly (e.g., periodically) at a sufficiently high rate. Inrefreshing, the memory device internally reads the data stored in agroup of memory cells and immediately rewrites the read data back to thesame memory cells. In some refreshing methods, the refreshing cycleincludes inversion of the stored data. Periodic refreshing withinversion assists in mitigating imprint as well as other impairmenttypes such as retention.

In some storage systems, the memory device internally encodes the datato be stored. In such memory devices, the refreshing operation includesdecoding the read data to correct readout errors, and re-encoding thedecoded data. When the refreshing operation also includes datainversion, parity bits that remain unchanged in the encoded inverteddata, are vulnerable to imprint effects. The logic required for decodingand re-encoding typically supports error correction of only one or twobits, and has large costs in terms of chip area, power consumption andrefreshing cycle latency.

A controller that stores data in the memory device is typically unawareof the internal refreshing and inversion carried out in the memorydevice. As a result, the polarity of data read from the memory may beeither un-inverted or inverted. To ensure reading the stored dataun-inverted, the memory device may track the polarity of the datare-written in the refreshing cycles, and upon a read request from thecontroller invert the read data as necessary. This approach, however, iscostly in terms of additional hardware required and increased powerconsumption in the memory device.

Embodiments that are described herein provide methods and systems forrecovering the data as stored, even though the actual polarity of theread data is unknown. In the disclosed embodiments, the data is storedencoded using an Error Correction Code (ECC) that generates for inputdata D multiple parity bits. The ECC is designed so that the parity bitscomprise disjoint bit-groups denoted P1 and P2 having different behaviorunder the inversion of the data to be encoded, as described herein.

Let P1(D) and P2(D) denote the parity bits values in the respectivegroups P1 and P2, resulting by encoding the data D in accordance withthe ECC. The parity bits in P1 result in the same values when encodingdata D or its inverse polarity version INV(D), i.e., P1(D)=P1[INV(D)].In contrast, the values of the parity bits in P2 have oppositepolarities in encoding D and INV(D), i.e., INV[P2(D)]=P2[INV(D)].

It can be shown that for an ECC that can be represented using aparity-check matrix H, the groups P1 and P2 correspond to rows in Hhaving an even or odd number of nonzero elements, respectively.

The ECC encoder output is referred to as a code word and is denoted CW.Using the above terminology, the code word can be expressed as CW=[D,P1(D),P2(D)].

As described above, the controller stores CW in the memory. Due to therefreshing cycles, however, the read data (denoted R) may haveun-inverted or inverted polarity, i.e., R=CW or, R=INV(CW),respectively, assuming the data is read with no errors.

Since CW is a valid code word of the underlying ECC, decoding R=CWrecovers the input data D. In addition, it can be shown that due to theproperty of the ECC regarding the parity bits as described above,decoding R′, which is derived from R by inverting the parity bits in thebit-group P1 of R, recovers INV(D). Note that in practical situationsthe data retrieved from the memory may contain errors. Successfuldecoding of D or INV(D) as described above is possible when the numberof errors is within the ECC capabilities. The decoding principlesdescribed above can be used to recover D in various ways, as will bedescribed in detail below.

In some embodiments, to recover the input data D, the controller appliesto the data read from the memory (with unknown polarity) two ECCdecoding operations. In a first ECC decoding operation the controllerattempts to decode the read data R as retrieved, and in a second ECCdecoding operation the controller attempts decoding R′, which equals theretrieved data R in which the parity bits in the group P1 have all beeninverted. Based on the pass/fail outcomes of the first and second ECCdecoding operations the controller identifies the actual polarity of theread data, and outputs the decoded data un-inverted or inverted,accordingly.

The controller may apply the ECC decoders and use their pass/failoutcomes to identify the polarity of the read data in various ways. Insome embodiments, the controller initially performs the first and secondECC decoding operations, e.g., in parallel. The controller then uses thepass/fail outcomes of both ECC decoding operations to identify thepolarity of the read data. For example, the controller identifies thepolarity according to successful decoding using one of the ECC decodingoperations and failure to decode using the other ECC decoding operation.In such embodiments, performing both the ECC decoding operations inparallel doubles the hardware required for decoding but results inminimal processing latency.

In other embodiments, the controller comprises only one ECC decoder,which decodes R and R′ in series. In an example embodiment, thecontroller starts with decoding R and if the decoding operation succeedsthe controller outputs the respective decoded data un-inverted. Ifdecoding R fails, the controller attempts decoding R′ and if thisdecoding operation succeeds the controller outputs the respectivedecoded data after inverting the decoded data.

In some embodiments, the memory device keeps track of the polarity ofthe stored data and the controller receives along with the read dataindication of the read data polarity. In such embodiments, thecontroller performs only one decoding operation, based on the indicatedpolarity.

In some embodiments, the ECC is designed such that for a code wordCW=[D, P(D)] that was written to the memory, wherein P(D) denotes theparity bits corresponding to data D, the ECC can decode both CW and itsinverse INV(CW)={INV(D),INV[P(D)]}. In such embodiments, the read datacan be recovered using a single decoding operation. Moreover, thepolarity of the decoded data can be determined by setting a bit in theinput data to a predefined value prior to encoding, and checking the bitvalue following the decoding.

In some embodiments, the memory device comprises a Multi-Level Cell(MLC) device that stores two or more bits per memory cell, by mappingthe bits to multiple respective analog values. The controller storesdata to the MLC device in units that are referred to as data pages,wherein a data page corresponds to a respective bit significance value.For example, in a 2bits/cell device, the controller may store (and read)a Least Significant Bit (LSB) page or a Most Significant Bit (MSB) page.

In some embodiments, the MLC device refreshes the memory cells byswapping among all the analog values in a predefined sequence. As willbe described in detail below, by carefully selecting the refreshingsequence, it can be guaranteed that the polarity of the entire data pageread from the memory is inverted or un-inverted. Therefore, thetechniques described above for recovering data with unknown polarity areapplicable also to memory devices that store multiple bits per cell.

In the disclosed techniques, strong ECC encoding/decoding operations arecarried out upon storing and reading the data rather than as part of therefreshing cycle. As a result, the inversion operation flips all thestored bits including all the parity bits, as required for efficientimprint mitigation. Moreover, by using the disclosed techniques, thepower consumption and latency of the refreshing cycles are reducedconsiderably.

SYSTEM DESCRIPTION

FIG. 1 is a block diagram that schematically illustrates a memory system20, in accordance with an embodiment that is described herein. System 20can be used in various host systems and devices, such as in computingdevices, cellular phones or other communication terminals, removablememory modules, Solid State Disks (SSD), Secure Digital (SD) cards,Multi-Media Cards (MMC) and embedded MMC (eMMC), digital cameras, musicand other media players and/or any other system or device in which datais stored and retrieved.

System 20 comprises a memory device 24, which stores data in a memorycell array 28. The memory array comprises multiple memory cells 32, suchas analog memory cells. In the context of the present patentapplication, the term “analog memory cell” is used to describe anymemory cell that holds a continuous, analog value of a physicalparameter, such as an electrical voltage or charge. In sometechnologies, such as, for example, technologies that employferroelectric materials, the memory cells store analog values of twoelectrical polarization states. Array 28 may comprise solid-state memorycells 32 of various kinds, such as, for example, phase change RAM (PRAM,also referred to as Phase Change Memory—PCM), Nitride Read Only Memory(NROM), Ferroelectric RAM (FRAM), Resistive RAM (ReRAM or PRAM), Intel'scross point memory technology (3D Xpoint™), magnetic RAM (MRAM) and/orDynamic RAM (DRAM) cells. Although the embodiments described hereinrefer mainly to analog memory, the disclosed techniques may also be usedwith various other memory types.

The charge levels stored in the cells and/or the analog voltages orcurrents written into and read out of the cells are referred to hereincollectively as analog values, storage values or analog storage values.Although the embodiments described herein mainly address thresholdvoltages, the methods and systems described herein may be used with anyother suitable kind of storage values, such as polarization states inFeRAMs.

System 20 stores data in the analog memory cells by programming thecells to assume respective memory states, which are also referred to asprogramming levels. The programming levels are selected from a finiteset of possible levels, and each level corresponds to a certain nominalstorage value. For example, a 1 bit/cell Single-Level Cell (SLC) can beprogrammed to assume one of two possible programming levels, and a 2bit/cell Multi-Level Cell (MLC) can be programmed to assume one of fourpossible programming levels by writing one of four possible nominalstorage values into the cell.

Memory device 24 comprises a reading/writing (R/W) unit 36, whichconverts data for storage in the memory device to analog storage valuesand writes them into memory cells 32. In alternative embodiments, theR/W unit does not perform the conversion, but is provided with voltagesamples, i.e., with the storage values for storage in the cells. Whenreading data out of array 28, R/W unit 36 converts the storage values ofmemory cells into digital samples having an integer resolution of one ormore bits. Data is typically written to and read from the memory cellsin data units that are referred to as data pages (or simply pages, forbrevity).

R/W unit 36 comprises a refresh and inversion module 38, whichrepeatedly (e.g., periodically) inverts the data stored in array 28. Thememory device carries out the refresh plus inversion cycles internally,at a sufficiently high rate, e.g., to mitigate imprint and retention. Asa result of the refreshing cycling, the polarity of the data that theR/W unit reads from array may be either un-inverted or inverted. In someembodiments, the R/W unit carries out a refreshing operation withoutinversion and a refreshing operation with inversion as separateoperations. For example, the R/W unit may refresh the data with noinversion at a higher rate than refreshing the data with inversion. Inother embodiments, the R/W unit carries out the refreshing plusinversion as a combined operation.

The storage and retrieval of data in and out of memory device 24 isperformed by a memory controller 40. Memory controller 40 comprises aninterface 44 for communicating with memory device 24, a processor 48,and an Error Correcting Code (ECC) module 50. The disclosed techniquescan be carried out in memory controller 40, by processor 48, ECC 50, orboth. Thus, in the present context, processor 48 and ECC 50 are referredto collectively as circuitry 54 that carries out the disclosedtechniques.

Memory controller 40 communicates with a host 52, for accepting data forstorage in the memory device and for outputting data retrieved from thememory device. ECC module 50 encodes the data for storage using asuitable ECC and decodes the ECC of data retrieved from the memory. ECCmodule 50 may comprise any suitable type of ECC, such as, for example,Low Density Parity Check (LDPC), Reed-Solomon (RS) orBose-Chaudhuri-Hocquenghem (BCH), can be used. In some embodiments, ECC50 implements a decoding scheme that resolves the unknown polarity ofthe data read from the memory. Detailed examples for such schemes areprovided further below.

Memory controller 40 may be implemented in hardware, e.g., using one ormore Application-Specific Integrated Circuits (ASICs) orField-Programmable Gate Arrays (FPGAs). Alternatively, the memorycontroller may comprise a microprocessor that runs suitable software, ora combination of hardware and software elements.

The configuration of FIG. 1 is an example system configuration, which isshown purely for the sake of conceptual clarity. Any other suitablememory system configuration can also be used. For example, although theexample of FIG. 1 shows a single memory device, in alternativeembodiments memory controller 40 may control multiple memory devices 24,e.g., in a RAID storage system. Elements that are not necessary forunderstanding the principles of the present disclosure, such as variousinterfaces, addressing circuits, timing and sequencing circuits anddebugging circuits, have been omitted from the figure for clarity.

In the example system configuration shown in FIG. 1, memory device 24and memory controller 40 are implemented as two separate IntegratedCircuits (ICs). In alternative embodiments, however, the memory deviceand the memory controller may be integrated on separate semiconductordies in a single Multi-Chip Package (MCP) or System on Chip (SoC), andmay be interconnected by an internal bus. Further alternatively, some orall of the memory controller elements may reside on the same die onwhich the memory array is disposed. Further alternatively, some or allof the functionality of memory controller 40 can be implemented insoftware and carried out by a processor (e.g., processor 48) or otherelement of the host system. In some embodiments, host 52 and memorycontroller 40 may be fabricated on the same die, or on separate dies inthe same device package.

In some embodiments, memory controller 40 comprises a general-purposeprocessor, which is programmed in software to carry out the functionsdescribed herein. The software may be downloaded to the processor inelectronic form, over a network, for example, or it may, alternativelyor additionally, be provided and/or stored on non-transitory tangiblemedia, such as magnetic, optical, or electronic memory.

In an example configuration of array 28, memory cells 32 are arranged inmultiple rows and columns, and each memory cell comprises afloating-gate transistor. In other embodiments, memory cells 32 compriseferroelectric cells that are arranged in multiple rows and columns, andeach memory cell has a 1T-1C architecture comprising one accesstransistor and one capacitor whose dielectric layer comprisesferroelectric material such as lead zirconate titanate (PZT). Such aferroelectric memory cell stores binary values “0” and “1” as one of twopossible electric polarizations.

The gates of the transistors in each row are connected by word lines,and the sources of the transistors in each column are connected by bitlines. In the present context, the term “row” is used in theconventional sense to mean a group of memory cells that are fed by acommon word line, and the term “column” means a group of memory cellsfed by a common bit line. The terms “row” and “column” do not connote acertain physical orientation of the memory cells relative to the memorydevice. The memory array is typically divided into multiple memorypages, i.e., groups of memory cells that are programmed and readsimultaneously.

To write data to a ferroelectric memory cell the R/W unit applies apositive or negative electrical field (e.g., voltage) across theferroelectric layer of the capacitor to write “0” or “1” depending onthe polarity of the electrical field.

To read data from a ferroelectric cell, the R/W unit applies anelectrical field of a given polarization, and senses a transientelectrical current of the cell capacitor. When the polarity of the fieldapplied matches the polarity to which the capacitor was charged, thesensed transient current would be low. When the polarity of the fieldapplied is opposite to the polarity to which the capacitor was charged,the sensed transient current would be significantly higher. As noted,reading the ferroelectric cell is destructive, and therefore the cellshould be re-written when the R/W unit senses a high current during theread operation.

In some embodiments, memory pages are sub-divided into sectors. Pagesmay be mapped to word lines in various manners. Each word line may storeone or more pages. A given page may be stored in all the memory cells ofa word line, or in a subset of the memory cells (e.g., the odd-order oreven-order memory cells).

ECC SCHEMES FOR RECOVERING DATA READ FROM MEMORY WITH UNKNOWN POLARITY

FIG. 2 is a block diagram that schematically illustrates an ErrorCorrection Code (ECC) module 50 that recovers data read from a memorydevice 24 with unknown polarity, in accordance with an embodiment thatis described herein. In the present example, memory device refreshes thememory cells by periodically inverting the data stored in the memorycells, e.g., to mitigate imprint effects. ECC 50 implements an encodingand decoding scheme that enables processor 48 to recover data stored inmemory device 24 even though the stored data is read with unknownpolarity.

ECC module 50 comprises an ECC encoder 60, which encodes input data 62(denoted D) into a code word 64 (denoted CW) in accordance with theunderlying ECC. In the present example, for protecting the data D, ECCencoder 60 generates multiple parity bits that are divided into disjointbit-groups denoted P1 and P2 that behave differently in encoding D orits inverse polarity version, as will be described in detail below. Notethat although the values of the parity bits P1(D) and P2(D) depend onthe data D, the manner in which the parity bits are divided among thebit-groups P1 and P2 depends on the underlying ECC but not on the dataD.

Code word 64, including data D and the respective parity bits P1(D) andP2(D) are stored in a given location of the memory device. In theexample of FIG. 2, for the sake of clarity, code word 64 is representedas CW=[D, P1(D), P2(D)], i.e., the input data D and the two groups ofparity bits P1 (D) and P2(D) are separated from one another. Inalternative embodiments, ECC encoder 60 can generate code words in whichD, P1(D) and P2(D) are interleaved with one another using any suitableinterleaving scheme, wherein the bit-groups are separable usingrespective suitable bit-masks.

As noted above, the bit-groups P1 and P2 behave differently for encodingdata with opposite polarities. For bit-group P1, the polarity of theparity bits is independent of the polarity of the input data, i.e.,

P1[INV(D)]=P1(D)  Equation 1:

and for bit-group P2, the polarity of the parity bits depends on thepolarity of the input data, i.e.,

P2[INV(D)] =INV [P2 (D)]  Equation2:

In Equations 1 and 2, the operator INV(•) denotes a bit-wise inversionor bit-flip operation, and P1(•) and P2(•) denote the parity bits ineach of the bit-groups P1 and P2 resulting from encoding input D orINV(D).

As seen in Equations 1 and 2, for bit-group P1, encoding D and INV(D)generate the same parity bits. In contrast, for bit-group P2, encoding Dand INV(D) results in parity bits that have opposite polarities. It canbe shown that for any linear ECC the generated parity bits (or partthereof) can be divided into bit-groups P1 and P2 having the propertiesdescribed above.

Assuming for the sake of simplicity ideal conditions in which the datastorage, refreshing and retrieval are all error-free, data 66 retrievedfrom the memory location (denoted R in the figure) can be either CW orINV(CW), because reading the memory location is not synchronized to therefreshing cycles in the memory device. When the polarity of the readdata is un-inverted, R is given by:

R=CW=[D, P1(D), P2(D)]  Equation 3:

and when the polarity of the read data is inverted, R is given by:

R=INV(CW)={INV(D), INV [P1(D)], INV [P2(D)]}  Equation 4:

ECC 50 comprises ECC decoders 70A and 70B that are both designed todecode code words generated by ECC encoder 60. Depending on theunderlying ECC, decoders 70A and 70B can correct up to a predefinednumber T of errors in the read data. When the input to decoder 70A or70B comprises a code word that contain up to T errors, the decoder candecode this code word successfully. When the number of errors exceeds T,the decoding fails.

Decoder 70A receives data R retrieved from the memory device, anddecodes R in accordance with the ECC that was used for encoding usingECC encoder 60. Decoder 70B receives an input denoted R′, which equalsthe read data R in which the parity bits in the bit-group P1 have beeninverted. When R=CW, R′ is given by:

R′ ={D, INV [P1(D)], P2(D)}  Equation 5:

and when R=INV(CW), R′ is given by:

R′={INV(D), P1(D), INV[P2(D)]}  Equation 6:

or using Equations 1 and 2 above, R′ in this case equivalently equals:

R′={INV(D), P1[INV(D)], P2[INV(D)]}  Equation 7:

Note that when R=CW, R in Equation 3 is a valid code word of the ECC,but R=INV(CW) in Equation 4 is generally not a valid code word of thisECC. Therefore ECC decoder 70A can decode R=CW successfully but wouldtypically fail to decode R=INV(CW).

Similarly, since R′ derived from R=INV(CW) in Equation 7 is a valid codeword of the ECC corresponding to input data INV(D), such R′ is decodableby decoder 70B. On the other hand, R′ resulting from R=CW in Equation 5is generally not a valid code word of the ECC and therefore cannot bedecoded successfully by decoder 70B. Table 1 below summarizes theconditions for decoding pass/fail outcome.

TABLE 1 ECC decoders possible pass/fail outcomes DATA READ FROM MEMORYDECODER-INPUT OUTCOME R = CW 70A-R Succeeds R = CW 70B-R′ Fails R = INV(CW) 70A-R Fails R = INV (CW) 70B-R′ Succeeds

As seen in Table 1, when reading from the memory device data withun-inverted polarity, decoder 70A succeeds to decode R=CW but decoder70B fails to decode the respective R′. Similarly, when reading from thememory device data having inverted polarity R=INV(CW), decoder 70A failsto decode R=INV(CW) and decoder 70B succeeds to decode the respectiveR′. In other words, at error-free conditions, the described decodingscheme results in only one of decoders 70A and 70B succeeding to decodeits respective input R or R′. When the read data contains errors, theprobabilities of decoders 70A and 70B to succeed in the decodingoperation depends on the error correction capability of the underlyingECC and on the ratio between the number of bits in the bit-groups P1 andP2.

ECC module 50 comprises decision logic 78 that receives decodingpass/fail outcomes as well as decoded data from decoders 70A and 70B.Decision logic 78 uses the decoding pass/fail indications to identifythe polarity of the data retrieved from the memory.

In some embodiments, to identify the polarity, logic 78 requirespass/fail outcomes from both decoders 70A and 70B. In such embodiments,ECC 50 executes both decoders (e.g., in parallel for minimal latency)before logic 78 decides on the polarity. In these embodiments, thepolarity decision is valid only if one decoder has succeeded and theother decoder has failed. In an embodiment, when both decoders fail orsucceed, logic 78 reports a readout failure.

In other embodiments, ECC 50 executes decoders 70A and 70B in series,i.e., ECC 50 first attempts decoding using one of the decoders, andexecutes the other decoder only if the first decoding fails. Forexample, starting with decoder 70A, if the decoding succeeds, logic 78decided that the polarity is un-inverted and skips the decoding usingdecoder 70B. Otherwise, ECC 50 executes decoder 70B, and if thisdecoding succeeds, logic 78 decides that the polarity is inverted. Whenboth decoders fail, logic 78 reports a readout failure.

Decision logic 78 recovers the data that was stored in the memory basedon the decided polarity. In an embodiment, when logic 78 decides thatthe polarity is un-inverted, logic 78 outputs the decoded data ofdecoder 70A, and when logic 78 decides that the polarity is inverted,logic 78 inverts the data decoded from decoder 78B using data inverter82 and outputs the inverted result.

As described above, determining the polarity of the read data depends onthe pass/fail results of decoders 70A and 70B. In some embodiments, toensure the same or similar decoding pass/fail probabilities in decoders70A and 70B, the ECC is designed so that the bit-groups P1 and P2comprise the same number of parity bits. Alternatively, the differencebetween the number of bits in the bit-groups P1 and P2 is smaller than apredefined difference threshold.

The configuration of ECC 50 in FIG. 2 is given by way of example, andother suitable ECC configurations can also be used. For example,although ECC 50 comprises two ECC decoders 70A and 70B, in alternativeembodiments, ECC 50 comprises a single ECC decoder, which is configuredto decode R and R′ in series.

FIG. 3 is a flow chart that schematically illustrates a method forrecovering data read from a memory device with unknown polarity, inaccordance with an embodiment that is described herein. The method isdescribed as being executed by processor 48. Alternatively, the methodcan be executed by circuitry 54 of memory controller 40, i.e., byprocessor 48, ECC 50 or in combination of both processor 48 and ECC 50.

The method begins with processor 48 encoding data for storage, at anencoding step 100. The data prior to encoding, denoted D, may beprovided by host 52 or originate within memory controller 40. Theprocessor encodes data D using an ECC that generates a code word CWcontaining D and multiple parity bits that are divided into bit-groupsP1 and P2 that satisfy Equations 1 and 2 above, in which P1(D) and P2(D)denote that parity bits in P1 and P2 corresponding to encoding D. Insome embodiments CW can be expressed as CW=[D, P1(D), P2(D)]. In otherembodiments, D, P1(D) and P2(D) in CW are interleaved with one anotherusing any suitable interleaving scheme. At a writing step 104, theprocessor stores the code word CW in a given location of the memorydevice.

At a retrieval step 108, the processor reads back the code word that wasstored in the given memory location. Since reading the stored data isnot synchronized to the refreshing and inversion cycles carried outinternally by the memory device, the stored data is read with unknownpolarity. Additionally, the data read may contain one or more errors.

At a first decoding step 112, the processor attempts to recover D bydecoding the code word CW from the read data. At a first verificationstep 116, the processor checks whether the first decoding has succeeded,and if so outputs the decoded data D, at an output step 120, and themethod terminates. Otherwise the first decoding has failed, and theprocessor attempts a second decoding to the read data, in which theparity bits in bit-group P1 are flipped, at a second decoding step 124.At a second verification step 128, the processor checks whether thesecond decoding has succeeded, and if so the processor inverts thedecoded data of the second decoding operation and outputs the invertedresult, at an inverted output step 132, and the method terminates.Otherwise, the second decoding has also failed, and the processorreports a readout failure, at a failure reporting step 136, and themethod then terminates.

In some embodiments, the memory device tracks the changes in thepolarity state of the data stored caused by the refreshing and inversioncycles. For example, the memory device may allocate for a group ofmemory cells a bit for storing inverted or un-inverted polarity statefor the entire group of the memory cells. In these embodiments, when thememory controller retrieves stored data, the memory controller receivesthe data together with the polarity indication, and performs only onedecoding operation based on the indication. If the polarity isun-inverted, the processor decodes the read data and outputs the decodeddata. Otherwise, the polarity is inverted, and the processor decodes theread data after flipping the parity bits in the bit-group P1 of the readdata, and outputs the decoded data inverted.

In some embodiments, ECC 50 is designed such that P[INV(D)]=P(D) for theentire parity bits P. In other words, the polarity of all the paritybits P is independent on the polarity of the data D, as specified forthe bit-group P1 in Equation 1. In such embodiments, the read data isdecodable, using a single decoding operation, regardless of itspolarity, as long as the number of errors in the read data is within theECC capabilities. In an embodiment, a bit of D is set to a predefinedvalue prior to encoding. In the decoded read data, the polarity of thisbit is checked to determine the polarity of the read data, i.e., whetherthe read data should be inverted or not.

Similar embodiments that require only a single decoding operation areapplicable for the case in which all the parity bits belong to thebit-group P2 as specified in Equation 2, i.e., when all the parity bitsP satisfy P[INV(D)]=INV[P(D)]. In this case the parity bits are invertedbefore decoding the read data.

EXTENSIONS TO MEMORY DEVICES THAT STORE MULTIPLE BITS PER MEMORY CELL

In the embodiments described above, we mainly assumed that memory device24 comprises a Single-Level Cell (SLC) device that stores 1bit/cell. Nowwe describe how the disclosed techniques are applicable also for memorydevices that store 2bits/cell or more.

Consider, for example, a Multi-Level Cell (MLC) device that stores2bits/cell. Such a device stores data in four analog values orprogramming levels that map bit-pairs as follows:

PV0: (1, 1)

PV1: (1, 0)

PV2: (0, 0)

PV3: (0, 1) wherein the left and right bits in each bit-pair, representsa Most Significant Bit (MSB) and a Least Significant Bit (LSB),respectively. Alternatively, other suitable mapping can also be used.

In some types of MLC devices of this sort, it is advantageous to refresha group of memory cells so that a group of the memory cells does notstore the same data for a long period of time. In some embodiments, arefreshing scheme re-writes the memory cells in the group so that eachcell is programmed in some sequence to all of the possible programminglevels, but retains each programming level for only a short period oftime. Similarly to the SLC case, reading the stored data is notsynchronized to the refreshing process and therefore upon read a givencell be programmed to any of the possible programming levels.

In the description that follows we describe a refreshing scheme thatguaranties that reading a LSB data page or a MSB data page results inreading the entire data page inverted or un-inverted. The refreshingscheme comprises two steps. In the first step, the level PV0 isexchanged with level PV3, and level PV1 is exchanged with level PV2. Inthe second step, the level PV0 is exchanged with level PV1, and levelPV2 is exchanged with level PV3. Refreshing using step 1 results in theinversion of the entire MSB page and does not change the polarity of therespective LSB page. Similarly, refreshing using step 2 flips thepolarity of the entire LSB page, and leaves the polarity of therespective MSB page unchanged. Table below summarizes the programminglevels swapping in step 1 and step 2.

TABLE 2 programming level swapping for refresh From level To level-STEP1To level-STEP2 PV0 PV3 PV1 PV1 PV2 PV0 PV2 PV1 PV3 PV3 PV0 PV2

By alternating between refreshing steps 1 and 2, each cell goes throughall the programming levels. For example, a cell that is initiallyprogrammed to PV0, goes through the sequence of programming levels PV0,PV3, PV2, PV1, PV0, and so on.

When the memory controller reads a LSB page or an LSB page from a MLCdevice that uses the refreshing scheme described above, the polarity ofthe entire data page is inverted or un-inverted, and therefore the datapage can be recovered using the embodiments described above for the SLCcase.

The described methods are applicable to higher capacity devices such asTLC devices that store 3bits/cell, or even higher capacity devices. Forexample, for a TLC device, the refreshing scheme comprises threerefreshing steps, wherein in each step a data page of one bitsignificance value may be inverted in its entirety.

The embodiments, described above are given by way of example, andalternative suitable embodiments can also be used.

In the example above, a group of memory cells stores multiple data pageshaving respective bit significance values, and the refreshing schemeinverts only one of the stored data pages in each refreshing cycle. Inalternative embodiments, the refreshing scheme may invert some or all ofthe data pages in the same refreshing cycle.

Although the disclosed embodiments refer mainly to mitigating imprinteffects, the disclosed techniques are applicable to memory devices ofany technology in which mitigating impairments of relevant typesinvolves refreshing the stored data with inversion.

Although the disclosed embodiments mainly refer to nonvolatile memories,the embodiments are applicable to volatile memories as well.

It will be appreciated that the embodiments described above are cited byway of example, and that the following claims are not limited to whathas been particularly shown and described hereinabove. Rather, the scopeincludes both combinations and sub-combinations of the various featuresdescribed hereinabove, as well as variations and modifications thereofwhich would occur to persons skilled in the art upon reading theforegoing description and which are not disclosed in the prior art.Documents incorporated by reference in the present patent applicationare to be considered an integral part of the application except that tothe extent any terms are defined in these incorporated documents in amanner that conflicts with the definitions made explicitly or implicitlyin the present specification, only the definitions in the presentspecification should be considered.

1. A memory controller, comprising: an interface configured tocommunicate with a memory device comprising multiple memory cells, thememory device refreshing the memory cells by repeatedly inverting datastored in the memory cells; and circuitry configured to: store inputdata in a group of the memory cells; read the stored input data from thegroup of the memory cells to produce read data, wherein due to therefreshing of the memory cells in the group, the read data has an actualpolarity that is either un-inverted or inverted; analyze the read datato identify the actual polarity of the read data; and recover the inputdata from the read data based on the identified actual polarity.
 2. Thememory controller according to claim 1, wherein the circuitry isconfigured to identify the actual polarity without receiving an explicitindication of the actual polarity from the memory device.
 3. The memorycontroller according to claim 1, wherein the circuitry is configured tostore the input data after encoding the input data using an ErrorCorrection Code (ECC) that generates first parity bits and second paritybits, such that encoding the input data and an inverted version of inputdata results in same first parity bits, but results in mutually-invertedsecond parity bits.
 4. The memory controller according to claim 3,wherein the circuitry is further configured to read the stored inputdata including the first and second parity bits, to perform at least oneof a first ECC decoding operation that decodes the read data directly,and a second ECC decoding operation that decodes the read data afterinverting the first parity bits, and to identify the actual polarity ofthe read data based on pass/fail outcomes of the at least one of thefirst and second ECC decoding operations.
 5. The memory controlleraccording to claim 4, wherein the circuitry is configured to decide thatthe actual polarity is un-inverted when the first ECC decoding operationsucceeds and the second ECC decoding operation fails, and to recover theinput data by outputting the data decoded by the first ECC decodingoperation.
 6. The memory controller according to claim 4, wherein thecircuitry is configured to decide that the actual polarity is invertedwhen the second ECC decoding operation succeeds and the first ECCdecoding operation fails, and to recover the input data by inverting thedata decoded by the second ECC decoding operation.
 7. The memorycontroller according to claim 4, wherein the circuitry is configured toreport a readout failure when both the first ECC decoding operation andthe second ECC decoding operation have failed, or when both the firstECC decoding operation and the second ECC decoding operation havesucceeded.
 8. The memory controller according to claim 4, wherein thecircuitry is configured to perform the second ECC decoding operationonly in response to a failure of the first ECC decoding operation. 9.The memory controller according to claim 1, wherein the memory cells inthe group store multiple data pages having respective bit significancevalues, and wherein the circuitry is configured to read the stored inputdata belonging to one of the data pages stored in the group, and toidentify the actual polarity and recover the stored input data of theone of the data pages.
 10. The memory controller according to claim 1,wherein the memory cells of the memory device comprise ferroelectricmemory cells.
 11. The memory controller according to claim 1, whereinthe circuitry is configured to set a given bit of the input data to apredefined value prior to encoding, to store the input data afterencoding the input data using an Error Correction Code (ECC) thatgenerates parity bits, such that encoding the input data and an invertedversion of input data results in same values of the parity bits, and toidentify the actual polarity by applying a single decoding operation tothe read data to produce decoded data, and checking the given bit in thedecoded data.
 12. A method, comprising: storing input data in a group ofmemory cells of a memory device, which applies refreshing to the memorycells by repeatedly inverting the data stored in the memory cells;reading the stored input data from the group of the memory cells toproduce read data, wherein due to the refreshing of the memory cells inthe group, the read data has an actual polarity that is eitherun-inverted or inverted; analyzing the read data to identify the actualpolarity of the read data; and recovering the input data from the readdata based on the identified actual polarity.
 13. The method accordingto claim 12, wherein identifying the actual polarity comprisesidentifying the actual polarity without receiving an explicit indicationof the actual polarity from the memory device.
 14. The method accordingto claim 12, wherein storing the input data comprises storing the inputdata after encoding the input data using an Error Correction Code (ECC)that generates first parity bits and second parity bits, such thatencoding the input data and an inverted version of input data results insame first parity bits, but results in mutually-inverted second paritybits.
 15. The method according to claim 14, wherein reading the storedinput data comprises reading the stored input data including the firstand second parity bits, wherein analyzing the read data comprisesperforming at least one of a first ECC decoding operation that decodesthe read data directly, and a second ECC decoding operation that decodesthe read data after inverting the first parity bits, and whereinidentifying the actual polarity of the read data comprises identifyingthe actual polarity of the read data based on pass/fail outcomes of theat least one of the first and second ECC decoding operations.
 16. Themethod according to claim 15, wherein identifying the actual polaritycomprises deciding that the actual polarity is un-inverted when thefirst ECC decoding operation succeeds and the second ECC decodingoperation fails, and wherein recovering the input data comprisesoutputting the data decoded by the first ECC decoding operation.
 17. Themethod according to claim 15, wherein identifying the actual polaritycomprises deciding that the actual polarity is inverted when the secondECC decoding operation succeeds and the first ECC decoding operationfails, and wherein recovering the input data comprises inverting thedata decoded by the second ECC decoding operation.
 18. The methodaccording to claim 15, wherein analyzing the read data comprisesreporting a readout failure when both the first ECC decoding operationand the second ECC decoding operation have failed, or when both thefirst ECC decoding operation and the second ECC decoding operation havesucceeded.
 19. The method according to claim 15, wherein analyzing theread data comprises performing the second ECC decoding operation only inresponse to a failure of the first ECC decoding operation.
 20. A storagesystem, comprising: a memory comprising multiple memory cells, thememory refreshing the memory cells by repeatedly inverting data storedin the memory cells; and a controller, which is configured to: storeinput data in a group of the memory cells; read the stored input datafrom the group of the memory cells to produce read data, wherein due tothe refreshing of the memory cells in the group, the read data has anactual polarity that is either un-inverted or inverted; analyze the readdata to identify the actual polarity of the read data; and recover theinput data from the read data based on the identified actual polarity.